Multi-Function Overlay Marks for Reducing Noise and Extracting Focus and Critical Dimension Information

ABSTRACT

An overlay mark includes a first, a second, a third, and a fourth component. The first component is located in a first region of the first overlay mark and includes a plurality of gratings that extend in a first direction. The second component is located in a second region of the first overlay mark and includes a plurality of gratings that extend in the first direction. The third component is located in a third region of the first overlay mark and includes a plurality of gratings that extend in a second direction different from the first direction. The fourth component is located in a fourth region of the first overlay mark and includes a plurality of gratings that extend in the second direction. The first region is aligned with the second region. The third region is aligned with the fourth region.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 17/816,030, filed on Jul. 29, 2022, entitled“MULTI-FUNCTION OVERLAY MARKS FOR REDUCING NOISE AND EXTRACTING FOCUSAND CRITICAL DIMENSION INFORMATION”, which is a continuation applicationof U.S. patent application Ser. No. 17/171,119, filed on Feb. 9, 2021,entitled “MULTI-FUNCTION OVERLAY MARKS FOR REDUCING NOISE AND EXTRACTINGFOCUS AND CRITICAL DIMENSION INFORMATION”, which is a divisionalapplication of U.S. patent application Ser. No. 16/026,309, filed onJul. 3, 2018, which claims benefit to U.S. Provisional PatentApplication No. 62/552,536, filed on Aug. 31, 2017, entitled“MULTI-FUNCTION OVERLAY MARKS FOR MEASURING FOCUS AND CRITICAL DIMENSION(CD) INFORMATION”, the disclosures of each which are incorporated hereinin their respective entireties.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. However, these advances haveincreased the complexity of processing and manufacturing ICs and, forthese advances to be realized, similar developments in IC processing andmanufacturing are needed. In the course of integrated circuit evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased.

Overlay marks have been used to measure the overlay or alignment betweenvarious layers of an IC. However, conventional overlay marks still haveshortcomings. For example, the measurement accuracy of a conventionaloverlay mark may be affected by the location of the overlay mark, suchas the pattern density of the IC features surrounding the overlay mark.As another example, while conventional overlay marks can be used tomeasure overlay, they have not been able to determine focus or criticaldimension (CD) information. Therefore, while existing overlay marks andhave been generally adequate for their intended purposes, they have notbeen entirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a schematic view of a lithography system constructed inaccordance with some embodiments of the present disclosure.

FIG. 2 is a sectional view of a EUV mask constructed in accordance withsome embodiments of the present disclosure.

FIG. 3 illustrates a simplified fragmentary cross-sectional view of anoverlay mark 100 in accordance with some embodiments of the presentdisclosure.

FIGS. 4A-4B illustrate superimposed simplified fragmentarycross-sectional views of an upper layer 130A and a lower layer 130B inaccordance with some embodiments of the present disclosure.

FIG. 5 illustrates a top view of an embodiment of an overlay mark on awafer in accordance with some embodiments of the present disclosure.

FIG. 6A is a simplified top view of a portion of a wafer in accordancewith some embodiments of the present disclosure.

FIG. 6B is a simplified top view of a test line that is verticallyoriented in accordance with some embodiments of the present disclosure.

FIG. 6C is a simplified top view of a test line that is horizontallyoriented in accordance with some embodiments of the present disclosure.

FIGS. 7A-7B each illustrate a cross-sectional profile of a plurality ofgratings of an overlay mark

FIGS. 8-10 illustrates a top view of an embodiment of an overlay mark ona wafer in accordance with some embodiments of the present disclosure.

FIG. 11 illustrates a plurality of overlay marks that are arranged as anM×N array around a main pattern in accordance with some embodiments ofthe present disclosure.

FIG. 12 illustrates top views of an upper layer and a lower layer of acomponent of an overlay mark in accordance with some embodiments of thepresent disclosure.

FIG. 13 illustrates top views of an upper layer and a lower layer of acomponent of an overlay mark in accordance with some embodiments of thepresent disclosure.

FIG. 14 illustrates a top view of an overlay mark in accordance withsome embodiments of the present disclosure.

FIG. 15A illustrates the fragmentary top views of an upper layer and alower layer of an FR region of an overlay mark in accordance with someembodiments of the present disclosure.

FIG. 15B illustrates the fragmentary top views of an upper layer and alower layer of an FL region of an overlay mark in accordance with someembodiments of the present disclosure.

FIG. 16 illustrates different graphs that correspond to an overlay markwith sub-patterns and an overlay mark without sub-patterns,respectively, in accordance with some embodiments of the presentdisclosure.

FIGS. 17A-17B illustrate fragmentary top views of portions of an overlaymark with sub-patterns in accordance with some embodiments of thepresent disclosure.

FIGS. 18A-18E illustrate the top views of several example embodiments ofsub-patterns in an overlay mark in accordance with some embodiments ofthe present disclosure

FIG. 19 illustrates a graph of critical dimension (CD) versus focus inaccordance with some embodiments of the present disclosure.

FIGS. 20A-20B illustrate a combo mark 700 and a compound mark 701,respectively, in accordance with some embodiments of the presentdisclosure.

FIGS. 21A, 21B, and 21C illustrate the top views of an overlay mark, inaccordance with some embodiments of the present disclosure.

FIG. 22 illustrates graphs that indicate the correlation betweendiffraction light intensity and CD, in accordance with some embodimentsof the present disclosure.

FIGS. 23-26 are flowcharts that illustrate the process flows associatedwith the overlay marks in accordance with some embodiments of thepresent disclosure.

FIGS. 27-32 illustrate cross-sectional side views of a device at variousstages of fabrication according to embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

To ensure accurate alignment (also referred to as overlay) between thevarious layers in a fabricated semiconductor device, overlay marks(which are different from alignment marks that are used align a maskwith a tool stage during lithography) are used to measure the alignmentbetween the layers. However, conventional overlay marks may haveshortcomings. For example, conventional overlay marks may be prone tobeing adversely affected by noise. As another example, conventionaloverlay marks may not be capable of extracting focus or criticaldimension information.

To overcome the problems discussed above, the present disclosureprovides embodiments of overlay marks that can reduce noise and alsocapable of extracting focus and critical dimension information. Thevarious aspects of the present disclosure will be discussed below ingreater detail with reference to FIGS. 1-24 . First, a EUV lithographysystem will be discussed below with reference to FIGS. 1-2 as an examplelithography context in which the overlay mark of the present disclosuremay be used, although it is understood that the overlay mark discussedherein may be used for other types of non-EUV lithography contexts too.Next, the details of the overlay mark according to embodiments of thepresent disclosure are discussed with reference to FIGS. 3-24 .

FIG. 1 is a schematic view diagram of a EUV lithography system 10,constructed in accordance with some embodiments. The EUV lithographysystem 10 may also be generically referred to as a scanner that isconfigured to perform lithography exposure processes with respectiveradiation source and exposure mode. The EUV lithography system 10 isdesigned to expose a photoresist layer by EUV light or EUV radiation.The photoresist layer is a material sensitive to the EUV light. The EUVlithography system 10 employs a radiation source 12 to generate EUVlight, such as EUV light having a wavelength ranging between about 1 nmand about 100 nm. In one particular example, the radiation source 12generates a EUV light with a wavelength centered at about 13.5 nm.Accordingly, the radiation source 12 is also referred to as EUVradiation source 12.

The lithography system 10 also employs an illuminator 14. In variousembodiments, the illuminator 14 includes various refractive opticcomponents, such as a single lens or a lens system having multiplelenses (zone plates) or alternatively reflective optics (for EUVlithography system), such as a single mirror or a mirror system havingmultiple mirrors in order to direct light from the radiation source 12onto a mask stage 16, particularly to a mask 18 secured on the maskstage 16. In the present embodiment where the radiation source 12generates light in the EUV wavelength range, the illuminator 14 employsreflective optics. In some embodiments, the illuminator 14 includes adipole illumination component.

In some embodiments, the illuminator 14 is operable to configure themirrors to provide a proper illumination to the mask 18. In one example,the mirrors of the illuminator 14 are switchable to reflect EUV light todifferent illumination positions. In some embodiment, a stage prior tothe illuminator 14 may additionally include other switchable mirrorsthat are controllable to direct the EUV light to different illuminationpositions with the mirrors of the illuminator 14. In some embodiments,the illuminator 14 is configured to provide an on-axis illumination(ONI) to the mask 18. In an example, a disk illuminator 14 with partialcoherence σ being at most 0.3 is employed. In some other embodiments,the illuminator 14 is configured to provide an off-axis illumination(OAI) to the mask 18. In an example, the illuminator 14 is a dipoleilluminator. The dipole illuminator has a partial coherence σ of at most0.3 in some embodiments.

The lithography system 10 also includes a mask stage 16 configured tosecure a mask 18. In some embodiments, the mask stage 16 includes anelectrostatic chuck (e-chuck) to secure the mask 18. This is because gasmolecules absorb EUV light, and the lithography system for the EUVlithography patterning is maintained in a vacuum environment to avoidthe EUV intensity loss. In the disclosure, the terms of mask, photomask,and reticle are used interchangeably to refer to the same item.

In the present embodiment, the lithography system 10 is a EUVlithography system, and the mask 18 is a reflective mask. One exemplarystructure of the mask 18 is provided for illustration. The mask 18includes a substrate with a suitable material, such as a low thermalexpansion material (LTEM) or fused quartz. In various examples, the LTEMincludes TiO2 doped SiO2, or other suitable materials with low thermalexpansion. In some embodiments, the LTEM includes 5%-20% by weight TiO2and has a thermal coefficient of expansion lower than about 1.0×10-6/°C. For example, in some embodiments, the TiO2 doped SiO2 material of theLTEM has a coefficient thermal expansion such that it varies by lessthan 60 parts-per-billion for every 1 degree Celsius of temperaturechange. Of course, other suitable materials having thermal coefficientof expansion that is equal to or less than TiO2 doped SiO2 may also beused.

The mask 18 also includes a reflective ML deposited on the substrate.The ML includes a plurality of film pairs, such as molybdenum-silicon(Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layerof silicon in each film pair). Alternatively, the ML may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configurable to highly reflect the EUV light.

The mask 18 may further include a capping layer, such as ruthenium (Ru),disposed on the ML for protection. The mask 18 further includes anabsorption layer deposited over the ML. The absorption layer ispatterned to define a layer of an integrated circuit (IC).Alternatively, another reflective layer may be deposited over the ML andis patterned to define a layer of an integrated circuit, thereby forminga EUV phase shift mask.

The lithography system 10 also includes a projection optics module (orprojection optics box (POB) 20 for imaging the pattern of the mask 18 onto a target 26 secured on a substrate stage 28 of the lithography system10. The POB 20 has refractive optics (such as for UV lithography system)or alternatively reflective optics (such as for EUV lithography system)in various embodiments. The light directed from the mask 18, diffractedinto various diffraction orders and carrying the image of the patterndefined on the mask, is collected by the POB 20. The POB 20 may includea magnification of less than one (thereby the size of the “image” on atarget (such as target 26 discussed below) is smaller than the size ofthe corresponding “object” on the mask). The illuminator 14 and the POB20 are collectively referred to as an optical module of the lithographysystem 10.

The lithography system 10 also includes a pupil phase modulator 22 tomodulate optical phase of the light directed from the mask 18 so thatthe light has a phase distribution on a projection pupil plane 24. Inthe optical module, there is a plane with field distributioncorresponding to Fourier Transform of the object (the mask 18 in thepresent case). This plane is referred to as projection pupil plane. Thepupil phase modulator 22 provides a mechanism to modulate the opticalphase of the light on the projection pupil plane 24. In someembodiments, the pupil phase modulator 22 includes a mechanism to tunethe reflective mirrors of the POB 20 for phase modulation. For example,the mirrors of the POB 20 are switchable and are controlled to reflectthe EUV light, thereby modulating the phase of the light through the POB20.

In some embodiments, the pupil phase modulator 22 utilizes a pupilfilter placed on the projection pupil plane. A pupil filter filters outspecific spatial frequency components of the EUV light from the mask 18.Particularly, the pupil filter is a phase pupil filter that functions tomodulate phase distribution of the light directed through the POB 20.However, utilizing a phase pupil filter is limited in some lithographysystem (such as an EUV lithography system) since all materials absorbEUV light.

As discussed above, the lithography system 10 also includes thesubstrate stage 28 to secure a target 26 to be patterned, such as asemiconductor substrate. In the present embodiment, the semiconductorsubstrate is a semiconductor wafer, such as a silicon wafer or othertype of wafer. The target 26 is coated with the resist layer sensitiveto the radiation beam, such as EUV light in the present embodiment.Various components including those described above are integratedtogether and are operable to perform lithography exposing processes. Thelithography system 10 may further include other modules or be integratedwith (or be coupled with) other modules.

The mask 18 and the method making the same are further described inaccordance with some embodiments. In some embodiments, the maskfabrication process includes two operations: a blank mask fabricationprocess and a mask patterning process. During the blank mask fabricationprocess, a blank mask is formed by deposing suitable layers (e.g.,reflective multiple layers) on a suitable substrate. The blank mask isthen patterned during the mask patterning process to achieve a desireddesign of a layer of an integrated circuit (IC). The patterned mask isthen used to transfer circuit patterns (e.g., the design of a layer ofan IC) onto a semiconductor wafer. The patterns can be transferred overand over onto multiple wafers through various lithography processes. Aset of masks is used to construct a complete IC.

The mask 18 includes a suitable structure, such as a binary intensitymask (BIM) and phase-shifting mask (PSM) in various embodiments. Anexample BIM includes absorptive regions (also referred to as opaqueregions) and reflective regions, patterned to define an IC pattern to betransferred to the target. In the opaque regions, an absorber ispresent, and an incident light is almost fully absorbed by the absorber.In the reflective regions, the absorber is removed and the incidentlight is diffracted by a multilayer (ML). The PSM can be an attenuatedPSM (AttPSM) or an alternating PSM (AltPSM). An exemplary PSM includes afirst reflective layer (such as a reflective ML) and a second reflectivelayer patterned according to an IC pattern. In some examples, an AttPSMusually has a reflectivity of 2%-15% from its absorber, while an AltPSMusually has a reflectivity of larger than 50% from its absorber.

One example of the mask 18 is shown in FIG. 2 . The mask 18 in theillustrated embodiment is a EUV mask, and includes a substrate 30 madeof a LTEM. The LTEM material may include TiO2 doped SiO2, and/or otherlow thermal expansion materials known in the art. In some embodiments, aconductive layer 32 is additionally disposed under on the backside ofthe LTEM substrate 30 for the electrostatic chucking purpose. In oneexample, the conductive layer 32 includes chromium nitride (CrN). Inother embodiments, other suitable compositions are possible, such as atantalum-containing material.

The EUV mask 18 includes a reflective multilayer (ML) structure 34disposed over the LTEM substrate 30. The ML structure 34 may be selectedsuch that it provides a high reflectivity to a selected radiationtype/wavelength. The ML structure 34 includes a plurality of film pairs,such as Mo/Si film pairs (e.g., a layer of molybdenum above or below alayer of silicon in each film pair). Alternatively, the ML structure 34may include Mo/Be film pairs, or any materials with refractive indexdifference being highly reflective at EUV wavelengths.

Still referring to FIG. 2 , the EUV mask 18 also includes a cappinglayer 36 disposed over the ML structure 34 to prevent oxidation of theML. In one embodiment, the capping layer 36 includes silicon with athickness ranging from about 4 nm to about 7 nm. The EUV mask 18 mayfurther include a buffer layer 38 disposed above the capping layer 36 toserve as an etching-stop layer in a patterning or repairing process ofan absorption layer, which will be described later. The buffer layer 38has different etching characteristics from the absorption layer disposedthereabove. The buffer layer 38 includes ruthenium (Ru), Ru compoundssuch as RuB, RuSi, chromium (Cr), chromium oxide, and chromium nitridein various examples.

The EUV mask 18 also includes an absorber layer 40 (also referred to asan absorption layer) formed over the buffer layer 38. In someembodiments, the absorber layer 40 absorbs the EUV radiation directedonto the mask. In various embodiments, the absorber layer may be made oftantalum boron nitride (TaBN), tantalum boron oxide (TaBO), or chromium(Cr), Radium (Ra), or a suitable oxide or nitride (or alloy) of one ormore of the following materials: Actinium, Radium, Tellurium, Zinc,Copper, and Aluminum.

The EUV lithography system discussed above in FIGS. 1-2 is merely anexample lithography system for which overlay marks can be used. However,the overlay marks of the present disclosure may be used for other typesof lithography systems too. The overlay marks of the present disclosurewill now be discussed below in more detail.

FIG. 3 illustrates a simplified fragmentary cross-sectional side view ofan overlay mark 100. The overlay mark 100 includes an upper layer 100Aand a lower layer 100B. In some embodiments, the upper layer 100Aincludes a patterned photoresist layer, and the lower layer 100Bincludes a patterned material layer on a wafer. In other embodiments,the upper layer 100A and the lower layer 100B may include differentpatterned layers on a wafer. It is understood that the upper layer 100Amay be patterned using one photomask, and the lower layer 100B may bepatterned using a different photomask.

The upper layer 100A and the lower layer 100B each include a pluralityof patterned components, also referred to as gratings. For example, theupper layer 100A includes a plurality of gratings 110A, and the lowerlayer 100B includes a plurality of gratings 110B. The gratings 110A and110B are elongated features that extend in a certain direction, forexample in a direction orthogonal to the cross-section in which thecross-sectional view of FIG. 3 is taken. In some embodiments, thegratings 110A are periodically distributed, and/or the gratings 110B areperiodically distributed. In other words, the gratings 110A areseparated from one another by a constant spacing, and the gratings 110Bare separated from one another by a constant spacing.

An overlay between the upper layer 100A and the lower layer 100B may bemeasured by light diffraction. For example, in response to incidentlight projected onto the overlay mark 100, different orders ofdiffracted light may be produced as a result. In FIG. 3 , a 0th orderdiffracted light is shown as I0, a +1 order diffracted light is shown asI₊₁, and a −1 order diffracted light is shown as I⁻¹. The intensities ofthe various diffracted orders of light may be measured by an opticalmeasurement tool. In some embodiments, the optical measurement toolincludes a scatterometry machine. In some other embodiments, the opticalmeasurement tool includes a diffractometry machine. It is understoodthat the optical measurement tool may also be configured to generate theincident light in some embodiments. Based on the measured I₊₁ and I⁻¹data, asymmetry information (As) associated with the overlay mark 100can be defined as As=I₊₁−I⁻¹.

The asymmetry information is used to determine overlay. For example, ifI⁻¹ is greater than I₊₁, it may indicate a misalignment between theupper and lower layers 100A and 100B where the upper layer 100A is“shifted to the left” compared to the lower layer 100B. If I₊₁ isgreater than I⁻¹, it may indicate a misalignment between the upper andlower layers 100A and 100B where the upper layer 100A is “shifted to theright” compared to the lower layer 100B. If I₊₁ is equal to I⁻¹, it mayrepresent a substantial alignment between the upper layer 100A and lowerlayer 100B. If the determined overlay does not meet specifications, thenthe upper layer 100A may be removed (e.g., via a photoresist strippingor ashing process), and a new upper layer may be formed or defined usingadjusted parameters (e.g. location of photomask). The new upper layermay still include a photoresist layer that is patterned by thephotomask, for example. The new upper layer should have better overlaywith the lower layer 100B as a result of the adjusted parameters.

Referring now to FIGS. 4A-4B, superimposed simplified fragmentarycross-sectional views of an upper layer 130A and a lower layer 130B areillustrated. The upper layer 130A and the lower layer 130B may be twodifferent layers of an overlay mark on a photomask. The upper layer 130Aand the lower layer 130B include a plurality of gratings 140A and 140B,respectively. A known bias may be introduced between the upper layer130A and the lower layer 130B. For example, FIG. 4A shows a known bias−d between the upper layer 130A and lower layer 130B. In other words,the gratings 140A in the upper layer 130A may be “shifted to the left”with respect to the gratings 140B in the lower layer 130B. On the otherhand, FIG. 4B shows a known bias +d between the upper layer 130A andlower layer 130B. In other words, the gratings 140A in the upper layer130A may be “shifted to the right” with respect to the gratings 140B inthe lower layer 130B. The known biases +d or −d may be intentionallyconfigured or implemented as a part of the design of the photomask.

As a wafer is fabricated, the actual total shift between an upper layerand a lower layer includes the known bias (+d or −d), as well as anoverlay (denoted herein as OV or OVL). The overlay refers to themisalignment between the upper and lower layers during waferfabrication, which is undesirable and should be minimized. Asymmetry isa function of the shift, and as such it is a function of the overlay,for example as expressed below: As=K*(OV+d), where As representsasymmetry, K represents a slope of a linear function (function ofasymmetry and shift), OV represents overlay, and d represents the knownbias. In this equation, As can be measured (e.g., by measuring thediffraction of light discussed above with reference to FIG. 3 ), and dis known. Thus, K and OV are two variables that can be calculated withtwo equations. For example, in the two equations below, As^(+d) andAs^(−d) are two different asymmetry calculations, one made with anoverlay mark where the upper and lower layers have a +d bias, and theother one made with an overlay mark where the upper and lower layershave a −d bias. Once the asymmetry information As^(+d) and As^(−d) iscalculated, the overlay OV and K can be calculated as well.

${\left. \begin{matrix}{{As}^{+ d} = {K^{*}\left( {{OV} + d} \right)}} \\{{As}^{- d} = {K^{*}\left( {{OV} - d} \right)}}\end{matrix} \right\}{OV}} = {{d \times \left( \frac{{As}^{+ d} + {As}^{- d}}{{As}^{+ d} - {As}^{- d}} \right)k} = \frac{{As}^{+ d} - {As}^{- d}}{2d}}$

Referring now to FIG. 5 , a top view of an embodiment of an overlay mark200A on a wafer is illustrated. The overlay mark 200A may include aplurality of components or portions, for example four differentcomponents 210-211 and 220-221 as shown in FIG. 5 . The components210-211 are aligned with each other, for example in the Y-direction, andthe components 220-221 are aligned with each other, for example in theY-direction.

The components 210-211 each include a plurality of gratings (e.g., suchas gratings 230) that extend in a Y-direction, and the components220-221 each include a plurality of gratings (e.g., such as gratings231) that extend in an X-direction that is perpendicular to theY-direction. Each of the components 210-211 and 220-221 also includes anupper layer and a lower layer, similar to the upper and lower layers100A and 100B shown in FIG. 3 . In each of the components 210-211 and220-221 of the overlay mark 200A, the gratings are located in both theupper layer and the lower layer.

A known bias is introduced in each of the components 210-211 and220-221. For the component 210, a known bias +d is introduced in theX-direction—that is, the gratings in its upper layer are offset from thegratings in its lower layer by +d in the X-direction. For the component211, a known bias −d is introduced in the X-direction—that is, thegratings in its upper layer are offset from the gratings in its lowerlayer by −d in the X-direction. For the component 220, a known bias −dis introduced in the Y-direction—that is, the gratings in its upperlayer are offset from the gratings in its lower layer by −d in theY-direction. For the component 221, a known bias +d is introduced in theY-direction—that is, the gratings in its upper layer are offset from thegratings in its lower layer by +d in the Y-direction. The components210-211 may be used to obtain two asymmetry measurements (e.g., As^(+d)and As^(−d)) in the X-direction, so as to be able to determine theoverlay in the X-direction. The components 220-221 may be used to obtaintwo asymmetry measurements (e.g., As^(−d) and As^(+d)) in theY-direction, so as to be able to determine the overlay in theY-direction.

According to the various aspects of the present disclosure, regions 240of the overlay mark 200A are located in a region with a lower patterndensity than regions 250 of the overlay mark 200A. In some embodiments,the lower pattern density region may include an edge of an IC, or anedge of a test line. For example, referring now to FIGS. 6A, 6B, and 6C,which illustrate some example situations where different patterndensities may exist. In more detail, FIG. 6A is a simplified top view ofa portion of a wafer 270 (e.g., an IC chip on the wafer), FIG. 6B is asimplified top view of a test line 271 that is vertically oriented(oriented in the Y-direction), and FIG. 6C is a simplified top view of atest line 272 that is horizontally oriented (oriented in theX-direction). For the portion of the wafer 270 in FIG. 6A, its edgeregions (such as edge regions 280-281) may have a lower pattern densitythan the rest of the portion of the wafer 270. Similarly, for the testlines 271-272, their edge regions (such as edge regions 282 and 283) mayhave a lower pattern density than the rest of the test lines too.

The differences in pattern density may lead to different profiles forpatterned features, such as the gratings in an overlay mark. An exampleof the different grating profiles as a result of the pattern densitydifference is shown in FIGS. 7A-7B. In more detail, FIG. 7A illustratesa cross-sectional profile of a plurality of gratings 300-303 of anoverlay mark, and FIG. 7B illustrates a cross-sectional profile of aplurality of gratings 310-313 of an overlay mark. The gratings 300-303and 310-313 may be embodiments of the gratings 230 or 231 discussedabove with reference to FIG. 5 . The gratings 300-303 and 310-313 areillustrated as “trenches” herein, but it is understood that they may be“islands” in other embodiments.

In FIG. 7A, the regions of the wafer (or test line) in which thegratings 300-303 are located may have relatively similar patterndensities. As such, the gratings 300-303 all have similarcross-sectional side view profiles, for example with respect to theirshapes, widths, or heights. In comparison, in FIG. 7B, the regions ofthe wafer (or test line) in which the gratings 310-313 are located mayhave different pattern densities. For example, the region of the wafer(or test line) in which the grating 310 is located may have a lowerpattern density than the regions of the wafer (or test line) in whichthe gratings 311-313 are located. This may occur when the grating 310 islocated close to an edge of the wafer or test line, such as in theregions 280-283 in FIGS. 6A-6C. As a result, the cross-sectional sideview profile of the grating 310 is different than the cross-sectionalside view profile of the gratings 311-313. Some of these differences mayinvolve differences in shape (e.g., more or less trapezoidal), or widthand/or height of the gratings. The differences in the gratings 310-313(due to pattern density differences) may lead to unwanted noise, whichcould adversely impact the overlay measurement accuracies.

Referring back to FIG. 5 , in order to suppress or minimize the effectsof this noise (caused by differences in pattern density) discussedabove, the present disclosure specifically arranges the components210-211 and 220-221 of the overlay mark 200A in a novel configuration.For example, since the regions 240 have lower pattern densities than theregions 250, the overlay mark 200A is arranged such that the components210-211 each have a border (e.g., the left border) that is located inthe regions 240, where these borders are aligned with each other. Stateddifferently, the components 210-211 are respectively arranged in the“top left” and “bottom left” corners of the overlay mark 200A in FIG. 5, and the components 220-221 are respectively arranged in the “topright” and “bottom right” corners of the overlay mark 200A in FIG. 5 .Note that the components 210-211 each have gratings that are oriented toextend in the Y-direction, with known biases +d and −d in theX-direction, respectively, whereas the components 220-221 each havegratings that are oriented to extend in the X-direction, with knownbiases +d and −d in the Y-direction.

The configuration of the components 210-211 and 220-221 shown in FIG. 5minimizes the noise discussed above. Since the components 210-211 eachhave gratings (e.g., the leftmost gratings) in the region 240 that hasthe lower pattern density, the components 210-211 will each havegratings in similar positions therein that have distortedcross-sectional profiles (compared to the gratings in the rest of thecomponents 210-211). As such, the components 210-211 will still havesimilar cross-sectional profiles as each other, since they each have oneor more gratings that are distorted as a result of being located in alow-pattern-density region. As discussed above, the overlay in theX-direction may be calculated using the components 210-211 of theoverlay mark 200A. In that case, the noise created as a result of thedistorted gratings in the low-pattern-density regions 240 may becanceled, since the components 210 and 211 each have the distortedgratings in the same or similar locations therein. Thus, the overlaymark 200A can achieve better overlay measurement accuracy thanconventional overlay marks.

FIGS. 8-10 illustrate top views of other embodiments of overlay marks,which are illustrated as overlay marks 200B, 200C, and 200D on a wafer,respectively. These embodiments of the overlay marks 200B, 200C, and200D are similar in some aspects to the embodiment of the overlay mark200A shown in FIG. 5 . Thus, for reasons of consistency and clarity,similar elements will be labeled the same in FIGS. 5 and 8-10 . Forexample, the overlay marks 200A, 200B, 200C, and 200D each includecomponents 210-211 with gratings that extend in the Y-direction, butwith +d and −d biases in the X-direction, respectively. The overlaymarks 200A, 200B, 200C, and 200D also each include components 220-221with gratings that extend in the X-direction, but with +d and −d biasesin the Y-direction, respectively. Furthermore, reference numerals 240are also used to designate regions with a low pattern density (e.g.,compared to the regions 250), and reference numerals 250 are also usedto designate regions with a high pattern density (e.g., compared withthe regions 240).

In the embodiment shown in FIG. 8 , the component 221 is implemented inthe “top left” corner of the overlay mark 200B, the component 220 isimplemented in the “bottom left” corner of the overlay mark 200B, thecomponent 211 is implemented in the “top right” corner of the overlaymark 200B, and the component 210 is implemented in the “bottom right”corner of the overlay mark 200B. As such, the components 210-211 arealigned in the Y-direction, and the components 220-221 are aligned inthe Y-direction.

Though the locations of the components 210-211 and 220-221 are differentcompared to the overlay mark 200A, both the overlay marks 200A and 200Bimplement two components adjacent to each other in the Y-direction (suchas components 210-211 being adjacent to each other, or components220-221 being adjacent to each other), where the adjacently-locatedcomponents each have gratings that extend in the same direction, be itthe X-direction (e.g., components 220-221) or the Y-direction (e.g.,components 210-211).

In the embodiment shown in FIG. 9 , the component 211 is implemented inthe “top left” corner of the overlay mark 200C, the component 221 isimplemented in the “bottom left” corner of the overlay mark 200C, thecomponent 210 is implemented in the “top right” corner of the overlaymark 200C, and the component 220 is implemented in the “bottom right”corner of the overlay mark 200C. As such, the components 210-211 arealigned in the X-direction, and the components 220-221 are aligned inthe X-direction.

Also different from the overlay marks 200A and 200B are the locations ofthe low pattern density regions 240 and the high pattern density regions250 of the overlay mark 200C. As shown in FIG. 9 , the topmost (in theY-direction) borders of the components 210-211 are located in the lowpattern density regions 240, and the bottommost (in the Y-direction)borers of the components 210-211—which are shared borders with thecomponents 221-220, respectively—are located in the pattern densityregion 250, which do not have low pattern density. For example, theregions 240 may be at or near an edge of an IC chip, or at or near anedge of a test line. Because of the specific configuration shown in FIG.9 , the noise introduced by the low pattern density regions 240 impactthe components 210-211 similarly, and thus the noise may be canceled outwhen the overlay calculations are done.

In the embodiment shown in FIG. 10 , the component 221 is implemented inthe “top left” corner of the overlay mark 200D, the component 211 isimplemented in the “bottom left” corner of the overlay mark 200D, thecomponent 220 is implemented in the “top right” corner of the overlaymark 200D, and the component 210 is implemented in the “bottom right”corner of the overlay mark 200D. As such, the components 210-211 arealigned in the X-direction, and the components 220-221 are aligned inthe X-direction.

The locations of the regions 240-250 are similar as in FIG. 9 . Notethat both the overlay marks 200C and 200D implement two componentsadjacent to each other in the X-direction (such as components 210-211being adjacent to each other, or components 220-221 being adjacent toeach other), where the adjacently-located components each have gratingsthat extend in the same direction, be it the X-direction (e.g.,components 220-221) or the Y-direction (e.g., components 210-211).

It is understood that additional embodiments of the overlay mark arealso possible. For example, in an alternative embodiment, the overlaymark may be arranged similarly as the overlay mark of 200A, but with thepositions of the components 220 and 221 swapped. As another example, inan alternative embodiment, the overlay mark may be arranged similarly asthe overlay mark of 200A, but with the positions of the components 210and 211 swapped. Regardless of the particular embodiment, the overlaymark of the present disclosure can achieve better overlay measurementperformance, as the arrangement discussed above allows the noise causedby the pattern density differences to be canceled or substantiallyreduced.

Referring now to FIG. 11 , a plurality of overlay marks 400 can bearranged as an M×N array around a main pattern 405 on a wafer accordingto various aspects of the present disclosure. M represents the number ofrows in the array, and N represents the number of columns in the array.In some embodiments, a main pattern 405 may be a portion of an IC, whichincludes patterns corresponding to active regions, source/drains,interconnect elements, etc. The main pattern 405 may include regionswith poor pattern density uniformity, for example, regions with muchgreater pattern densities than other regions. The main pattern 405 mayalso include regions of bad CD uniformity, for example, regions withgreater CD differences.

The M×N array of overlay marks 400 may include a plurality of overlaymarks, such as the eight overlay marks 400A-400H illustrated in FIG. 11. At least some of these overlay marks 400A-400H may include multiplecomponents that are similar to the components 210-211 and/or 220-221discussed above. For example, the overlay marks 400A, 400B, 400C, and400D are disposed on 4 different sides (e.g., top, bottom, left, andright) of the main pattern 405. The overlay marks 400A-400D each includea component 410 similar to the component 210 discussed above, acomponent 411 similar to the component 211 discussed above, a component420 similar to the component 220 discussed above, and a component 421similar to the component 221 discussed above. For example, each of thesecomponents 410-411 and 420-421 may include an upper layer and a lowerlayer, where a bias of +d or a bias of −d exists between the upper andlower layers. Again, in some embodiments, the upper layer may include apatterned photoresist layer while the lower layer may include apatterned material layer on a substrate. In other embodiments, the upperlayer and the lower layer may include different patterned layers on asubstrate. Similar to the components 210-211 and 220-221, the components410-411 are configured to measure the X-direction overlay, and thecomponents 420-421 are configured to measure the Y-direction overlay.

For reasons similar to those discussed above with reference to FIGS. 5and 8-10 , the components in the overlay marks 400A-400B (which eachface the X-direction borders of the main pattern 405) are configuredsuch that the two components 420-421 (for measuring Y-directionaloverlay) are arranged adjacent to one another in the X-direction, andthe two components 410-411 (for measuring X-directional overlay) arearranged adjacent to one another in the X-direction. As discussed above,this type of arrangement ensures that whatever noise that may come in atthe horizontal edges of the overlay marks 400A-400B (the edges that facethe “top” and “bottom” borders of the main pattern 405) will be reducedor canceled, when overlay is being measured using asymmetry informationregarding diffracted light intensity.

Also for reasons similar to those discussed above with reference toFIGS. 5 and 8-10 , the components in the overlay marks 400C-400D (whicheach face the Y-direction borders of the main pattern 405) areconfigured such that the two components 420-421 (for measuringY-directional overlay) are arranged adjacent to one another in theY-direction, and the two components 410-411 (for measuring X-directionaloverlay) are arranged adjacent to one another in the Y-direction. Again,this type of arrangement ensures that whatever noise that may come in atthe vertical edges of the overlay marks 400C-400D (the edges that facethe “left” and “right” borders of the main pattern 405) will be reducedor canceled, when overlay is being measured using asymmetry informationregarding diffracted light intensity.

Whereas the overlay marks 400A-400D are each disposed adjacent to arespective border of the main pattern 405, the overlay marks 400E-400Hare each disposed diagonally adjacent to a respective corner of the mainpattern 405. For example, the overlay mark 400E is disposed diagonallyadjacent to the “top left” corner of the main pattern 405, the overlaymark 400F is disposed diagonally adjacent to the “top right” corner ofthe main pattern 405, the overlay mark 400G is disposed diagonallyadjacent to the “bottom left” corner of the main pattern 405, and theoverlay mark 400H is disposed diagonally adjacent to the “bottom right”corner of the main pattern 405.

Due to the diagonal locations of the overlay marks 400E-400H, theircomponents are configured differently. For example, the overlay marks400E-400H also include components 410-411 that are similar to thecomponents 210-211 discussed above. However, the components 410-411 arediagonally disposed with respect to each other, rather than beingdisposed immediately adjacent to one another in the X-direction or inthe Y-direction. In addition, the overlay marks 400E-400H each includecomponents 430A-430B, as well as components 470A-470B.

The components 430A-430B are configured to measure overlay in theY-direction, and as such they each include gratings that extend in theX-direction. In this manner, the components 430A-430B collectively servea function that is similar to the component 420 (or component 421). Inother words, the components 430A-430B are similar to the component 420being divided into two portions, where the divided left portionresembles the component 430A, and the divided right portion resemblesthe component 430B. However, unlike the component 420—where a constantbias d (be it plus or minus) exists in the Y-direction between its upperand lower layers—the gratings in the components 430A and 430B havedifferent biases. In some embodiments, the upper layer may include apatterned photoresist layer while the lower layer may include apatterned material layer on a substrate. In other embodiments, the upperlayer and the lower layer may include different patterned layers on asubstrate.

For example, referring to FIG. 12 , top views of the upper layer and thelower layer of the components 430A-430B are illustrated separatelyaccording to embodiments of the present disclosure. The components 430Aand 430B share a border 440 that extends in the Y-direction. This border440 may be conceptual and not visible in the actual overlay mark. Inother words, the border 440 merely denotes what would have been thedemarcation line between the components 430A and 430B.

In the illustrated embodiment, the horizontally-oriented gratings in thecomponent 430A are biased with +d, meaning that the gratings between theupper layer and the lower layer of the component 430A are shifted in theY-direction by a distance +d. Meanwhile, the horizontally-orientedgratings in the component 430B are biased with −d, meaning that thegratings between the upper layer and the lower layer of the component430B are shifted in the Y-direction by a distance −d. In the illustratedembodiment, the different biases between the components 430A and 430Bare reflected as a shifting of the gratings in the upper layer, but thegratings in the lower layer of the components 430A and 430B remainun-shifted. However, this is merely an example. In other embodiments,the gratings in the upper layer may be un-shifted, while the gratings inthe lower layer are shifted by +d and −d for the components 430A and430B, respectively.

Referring back to FIG. 11 , the components 470A-470B are also configuredto measure overlay in the Y-direction, and as such they each includegratings that extend in the X-direction. In this manner, the components470A-470B collectively serve a function that is similar to the component420 (or the component 421). In other words, the components 470A-470B aresimilar to the component 420 being diagonally divided into two portions,where one diagonal portion (shaped as a right triangle) resembles thecomponent 470A, and the opposite diagonal portion (shaped as anotherright triangle) resembles the component 470B. However, unlike thecomponent 420—where a constant bias d (be it plus or minus) exists inthe Y-direction between its upper and lower layers—the gratings in thecomponents 470A and 470B have different biases.

For example, referring to FIG. 13 , top views of the upper layer and thelower layer of the components 470A-470B are illustrated separatelyaccording to embodiments of the present disclosure. The components 470Aand 470B share a border 480 that extends diagonally in both theX-direction and the Y-direction. This border 480 may be conceptual andnot visible in the actual overlay mark. In other words, the border 480merely denotes what would have been the demarcation line between thecomponents 470A and 470B.

Note that since the components 470A-470B are shaped similar totriangles, the gratings located in each of the components 470A-470B haveuneven lengths in the X-direction. In the illustrated embodiment, thehorizontally-oriented gratings in the component 470A are biased with +d,meaning that the gratings between the upper layer and the lower layer ofthe component 470A are shifted in the Y-direction by a distance +d.Meanwhile, the horizontally-oriented gratings in the component 470B arebiased with −d, meaning that the gratings between the upper layer andthe lower layer of the component 470B are shifted in the Y-direction bya distance −d. In the illustrated embodiment, the different biasesbetween the components 470A and 470B are reflected as a shifting of thegratings in the upper layer, but the gratings in the lower layer of thecomponents 470A and 470B remain un-shifted. However, this is merely anexample. In other embodiments, the gratings in the upper layer may beun-shifted, while the gratings in the lower layer are shifted by +d and−d for the components 470A and 470B, respectively. It is also understoodthat although the components 470A and 470B are implemented in each ofthe overlay marks 400E, 400F, 400G, and 400H, they may be rotated indifferent directions in different overlay marks.

For reasons similar to those discussed above, the configuration of thecomponents 430A-430B and 470A-470B also suppresses noise caused by themain pattern 405's pattern density issues. For example, theconfiguration of the components 430A-430B and 470A-470B allows thegratings for measuring the same direction's overlay to be located atsimilar distances to the main pattern 405. Thus, even if the mainpattern 405's pattern density (or CD uniformity) issues cause noise thatmay come in at the borders of the overlay marks 400E-400H, the noisewill be substantially reduced.

In the embodiment shown in FIGS. 11-13 . The divided components (e.g.,components 430A-430B and 470A-470B) in the overlay marks have gratingsthat extend in the X-direction, so that they are configured to measurethe Y-directional overlay. However, this is merely an example. It isalso understood that in some other embodiments, the divided componentsmay have gratings that extend in the Y-direction, so that they areconfigured to measure the X-directional overlay. It is also understoodthat the configuration of the M×N array of overlay marks 400 herein isnot limited to the eight overlay marks 400A-400H discussed above. Inother embodiments, additional overlay marks similar to those discussedabove may be implemented in each row and/or each column of the array.

FIG. 14 illustrates a top view of an overlay mark 500 according toanother embodiment of the present disclosure. The overlay mark 500 canbe used to not only measure overlay, but also focus information, forexample a focus that leads to an optimal critical dimension (CD) forsemiconductor features on a wafer, as discussed in more detail below.

As is shown in FIG. 14 , the overlay mark 500 may include components510-511 that are used to measure an X-directional overlay, similar tothe components 210-211 discussed above. The overlay mark 500 may alsoinclude components 520-521 that are used to measure a Y-directionaloverlay, similar to the components 220-221 discussed above. Thecomponent 510 is arranged at the “top left” corner of the overlay mark500, the component 511 is arranged at the “bottom right” corner of theoverlay mark 500, the component 520 is arranged at the “top right”corner of the overlay mark 500, and the component 521 is arranged at the“bottom left” corner of the overlay mark 500. As such, the component 510is located directly adjacent to the components 520-521 (and the same istrue for the component 511), and the components 510 and 511 are alsolocated diagonally adjacent to one another. However, it is understoodthat the arrangement of the components 510-511 and 520-521 shown in FIG.14 is merely an example, and that they may be arranged differently inother embodiments, for example similar to the embodiments shown in FIGS.5 and 8-10 .

It is also understood that the components 510-511 and 520-521 may eachhave a built-in bias with respect to its upper layer and lower layer,similar to the overlay marks discussed above with reference to FIGS. 5and 8-10 . For example, the component 510 may have a +d bias between itsupper and lower layers in the X-direction, the component 511 may have a−d bias between its upper and lower layers in the X-direction, thecomponent 520 may have a −d bias between its upper and lower layers inthe Y-direction, and the component 521 may have a +d bias between itsupper and lower layers in the Y-direction. Again, in some embodiments,the upper layer may include a patterned photoresist layer while thelower layer may include a patterned material layer on a substrate. Inother embodiments, the upper layer and the lower layer may includedifferent patterned layers on a substrate.

As shown in FIG. 14 , each of the components 510-511 and 520-521 isdivided into a region FR and a region FL, where the regions FR and FLeach include one or more sub-patterns (discussed below in more detailbelow). In the illustrated embodiment, the FR regions in the components510 and 511 are oriented perpendicular to the FR regions in thecomponents 520-521. Likewise, the FL regions in the components 510 and511 are oriented perpendicular to the FL regions in the components520-521. The details of the region FR and the region FL is illustratedin FIGS. 15A-15B as examples. Specifically, FIG. 15A illustrates thefragmentary top views of an upper layer and a lower layer of the regionFR (e.g., the region FR from the component 520), and FIG. 15Billustrates the fragmentary top views of an upper layer and a lowerlayer of the region FL (e.g., the region FL from the component 520).

Referring to FIG. 15A, the upper layer of the region FR includes aplurality of elongated gratings 530 and a plurality of sub-patterns 540.The elongated gratings may extend in the X-direction or in theY-direction, depending on component of the overlay mark 500 in whichthey are located. In the illustrated embodiment, the gratings 530located in the components 510-511 (for measuring X-direction overlay)extend in the Y-direction, and the gratings 530 located in thecomponents 520-521 (for measuring Y-direction overlay) extend in theX-direction. For example, the gratings 530 in the FR region of thecomponent 510 are perpendicular to the gratings 530 in the FR region ofthe component 520.

The sub-patterns 540 include patterns that are significantly smallerthan the gratings 530 in size. In some embodiments, the difference insize between each of the gratings 530 and each of the sub-patterns 540is at least one order of magnitude (e.g., ×10). The sub-patterns 540 areadded to obtain focus information, as discussed in more detail below. Inthe embodiment illustrated in FIG. 15A, the sub-patterns 540 are dividedinto several subsets, where the sub-patterns 540 in each subset arearranged as a “column” that is located to the “right” (in theX-direction) of a respective one of the vertically-oriented gratings530. It is understood that, for the FR region located in the components520 or 521, the gratings 530 would be rotated 90 degrees so as to extendin the X-direction, and the sub-patterns 540 would be divided intosubsets where each subset is arranged as a “row” that is located to the“top” or “bottom” (in the Y-direction) of a respective one of thehorizontally-oriented gratings 530 in that case.

Still referring to FIG. 15A, the lower layer of the region FR includes aplurality of elongated gratings 550. The gratings 550 extend in the samedirection in which the gratings 530 extend (e.g., the Y-direction in thecomponents 510-511 and the X-direction in the components 520-521).However, there are no sub-patterns in the lower layer in thisembodiment. It is understood, however, that in some other embodiments,the sub-patterns 540 may be located in the lower layer instead of theupper layer.

Referring now to FIG. 15B, the region FL shares many similarities withthe region FR. For example, both the regions FR and FL include an upperlayer and a lower layer. A plurality of elongated gratings 530 areimplemented in the upper layer, and a plurality of elongated gratings550 are implemented in the lower layer. In addition, for both theregions FR and FL, one of the layers (e.g., the upper layer herein)includes a plurality of sub-patterns 540, but the other layer (e.g., thelower layer herein) does not. Unlike the region FR, however, thesub-patterns 540 are located to the “left” (in the X-direction) of thegratings 530. Had the region FR been implemented in the components520-521 (e.g., for measuring the Y-direction overlay), the sub-patterns540 would be located to the “bottom” or “top” (in the Y-direction) ofthe gratings 530. In this manner, the regions FR and FL are symmetricalto one another.

The presence of the sub-patterns 540 in the regions FR or FL affects thediffraction intensity signal, such that a “side lobe” is manifested.This is illustrated in FIG. 16 , which includes a graph 570 and a graph571 that correspond to an overlay mark with the sub-patterns (such asthe sub-patterns 540) and an overlay mark without the sub-patterns,respectively. In more detail, graph 570 and graph 571 each include anX-axis that represents a shift in position, which may include overlayand the built-in bias between an upper layer and a lower layer. Graph570 and graph 571 also each include a Y-axis that represents adiffraction light intensity, which may correspond to the asymmetrybetween the +1^(st) order diffracted light and the −1^(st) orderdiffracted light. For example, the asymmetry may include the asymmetryinformation (denoted as As=I₊₁−I⁻¹) discussed above.

Thus, the graphs 570 and 571 illustrate how the diffraction lightintensity (e.g., As=I₊₁−I⁻¹) varies as a function of a positional shift(which includes the overlay) between an upper layer and a lower layer ofan overlay mark with the sub-patterns and an overlay mark without thesub-patterns, respectively. However, the presence of the sub-patterns(such as the sub-patterns 540 in FIGS. 15A-15B) leads to side-lobes inthe graph 570, for example side-lobes 580 and 581. The side-lobes 580and 581 manifest themselves as a “bump” or “protrusion” in what shouldotherwise be a smooth curve for the graph 570. In other words, the graph570 would substantially resemble the graph 571, had the sub-patterns 540not been implemented.

In some embodiments, the side-lobe 580 may be caused by the sub-patterns540 from the region FR, and the side-lobe 581 may be caused by thesub-patterns 540 from the region FL. The side-lobes 580-581 areassociated with focus information (focus of the exposure in alithography process), and as such they may be used to determine what theoptimal focus is for achieving good critical dimension. Since theside-lobes 580-581 are symmetrical with each other, they can be canceledout for purposes of determining overlay. In this manner, the overlaymark (such as the overlay mark 500) of the present disclosure may beused to obtain both overlay and focus information.

FIGS. 17A-17B illustrate fragmentary top views of portions of an overlaymark with sub-patterns according to embodiments of the presentdisclosure. Various parameters that may define the sub-patterns (e.g.,the number and position of the sub-patterns) are also visuallyillustrated in FIGS. 17A-17B. In FIG. 17A, a plurality of rectangularsub-patterns 540 are implemented adjacent to the gratings 530. Thenumber of the sub-patterns 540 is denoted by a parameter N, where N>=1.

The sub-patterns may also be implemented around the gratings 530 in morethan one direction. For example, as shown in FIG. 17B, one or moresub-patterns 540A may be arranged to the “left” side of the grating 530.The parameter Nv>=1 indicates that there are 1 or more of thesub-patterns 540A in this region. The sub-patterns may be arranged as asingle column/row, or an array with multiple columns and/or multiplerows. The parameter I_(v) represents a distance between the grating 530and the nearest one of the sub-patterns 540A. The parameter P_(v)represents a pitch (e.g., a sum of a dimension of one of thesub-patterns and a distance between of two of the adjacent sub-patterns540A) of the sub-patterns 540A in the Y-direction. The parameter P_(vh)represents another pitch of the sub-patterns in the X-direction, forexample when there two or more columns (columns that extend in theY-direction) of the sub-patterns 540A. The parameter P_(v) is measuredin the Y-direction, while the parameter P_(vh) is measured in theX-direction.

Another group of the sub-patterns 540B are located “directly above” thegrating 530 in the Y-direction. The parameter Nhh>=0 means that thenumber of the sub-patterns 540B in the “region above” the grating 530may be 1 or more (if Nhh>0), or none at all (if Nhh=0). The parameterInn represents a distance between the grating 530 and the nearest one ofthe sub-patterns 540B. The parameter P_(hh) represents a pitch of thesub-patterns 540B in the Y-direction.

Another group of the sub-patterns 540C are located “directly below” thegrating 530 in the Y-direction. The parameter Nhl>=0 means that thenumber of the sub-patterns 540C in the “region below” the grating 530may be 1 or more (if Nhl>0), or none at all (if Nhl=0). The parameterI_(hl) represents a distance between the grating 530 and the nearest oneof the sub-patterns 540C. The parameter P_(hl) represents a pitch of thesub-patterns 540C in the Y-direction.

Another group of the sub-patterns 540D are located “diagonally above”the grating 530 in the X-direction and in the Y-direction. The parameterNch>=0 means that the number of the sub-patterns 540D in the “regiondiagonally above” the grating 530 may be 1 or more (if Nch>0), or noneat all (if Nch=0). The parameter I_(ch) represents a diagonal distancebetween the grating 530 and the nearest one of the sub-patterns 540D inthe X and Y directions. The parameter P_(ch) represents a diagonal pitchof the sub-patterns 540D in the X and Y directions. An angle θ_(ch)represents the diagonal angle of the sub-patterns 540D.

Another group of the sub-patterns 540E are located “diagonally below”the grating 530 in the X-direction and in the Y-direction. The parameterNcl>=0 means that the number of the sub-patterns 540E in the “regiondiagonally below” the grating 530 may be 1 or more (if Ncl>0), or noneat all (if Ncl=0). The parameter I_(cl) represents a diagonal distancebetween the grating 530 and the nearest one of the sub-patterns 540E inthe X and Y directions. The parameter P_(cl) represents a diagonal pitchof the sub-patterns 540E in the X and Y directions. An angle θ_(cl)represents the diagonal angle of the sub-patterns 540E.

The configuration of the sub-patterns 540A-540E may be used to fine tunea lithography process window, for example the focus or exposure dose.The configuration of the sub-patterns 540A-540E may also be used toadjust the side-lobe effect discussed above and/or modify a Bossungcurve discussed below.

Although the embodiments discussed above have shown the sub-patterns 540as having somewhat rectangular top-view shapes, other designs arepossible for the sub patterns. For example, referring to FIGS. 18A-18E,the top views of several example embodiments of the sub-patterns areillustrated.

In the embodiment shown in FIG. 18A, sub-patterns 540F are implementedadjacent to the gratings 530. In this embodiment, the sub-patterns 540Finclude five polygons. Each polygon has three or more sides (i.e., n>=3,where n is the side of the polygon). The number of sides of the polygonmay be configured to adjust the side-lobe effect produced by thegratings diffraction. With different polygons, the overlay mark willproduce slightly different depth of focus (DOF).

In the embodiment shown in FIG. 18B, sub-patterns 540G are implementedadjacent to the gratings 530. In this embodiment, the sub-patterns 540Ginclude four circles or circular-shaped patterns. The sub-patterns 540Gmay also be referred to as point symmetry sub-patterns. The number,size, and/or location of the circular shaped sub-patterns 540G may beconfigured to adjust the side-lobe effect smoothly.

In the embodiment shown in FIG. 18C, sub-patterns 540H are implementedadjacent to the gratings 530. In this embodiment, the sub-patterns 540Hinclude five ellipse-like patterns. The sub-patterns 540H may also bereferred to as axis symmetry sub-patterns. The number, size, and/orlocation of the ellipse-like sub-patterns 540H may also be configured toadjust the side-lobe effect smoothly.

In the embodiment shown in FIG. 18D, sub-patterns 540I are implementedadjacent to the gratings 530. In this embodiment, the sub-patterns 540Iare shaped as “lines”, having top view shapes that are similar to thegratings 530. One or more of the “line-like” sub-patterns 540I may beimplemented on either side of the grating 530. Parameters such ashorizontal/vertical dimensions of each sub-pattern 540I, a spacingbetween the grating 530 and the nearest sub-pattern 540I, and/or a pitchof the sub-patterns 540I may be configured to adjust the side-lobeeffect.

In the embodiment shown in FIG. 18E, combinations of different types ofsub-patterns may be implemented adjacent to the gratings 530. Forexample, the line-like sub-patterns 540I may be implemented adjacent tothe gratings 530, and the rectangular sub-patterns 540A may beimplemented adjacent to the line-like sub-patterns 540I. In otherembodiments, the sub-patterns 540A, 540F, 540G, 540H, 540I may be mixedinto various different combinations and sub-combinations. In addition,any one of the shapes of the sub-patterns discussed herein may be usedto implement the sub-patterns 540B-E that are disposed on differentdirections of the grating 530, as shown in FIG. 17B.

It is also understood that the shapes and profiles of the gratings andsub-patterns discussed above in the present disclosure may represent thedesign on a photomask. However, once these patterns are patterned onto awafer, they may lose their resemblance to their original designs, due tooptical effects. For example, patterns having rectangular or polygonaloriginal designs may have more rounded or at least non-straight edgesonce they are patterned onto a wafer. Nonetheless, they may still causethe side-lobes discussed above, which will allow the overlay marksherein to be used to determine both overlay and focus.

Referring back to FIG. 14 , the components 510-511 are each used tomeasure the X-direction overlay, and the components 520-521 are eachused to measure Y-direction overlay. As such, the regions FR and FL inthe components 510-511 are disposed adjacent to one another in theY-direction, and the regions FR and FL in the components 520-521 aredisposed adjacent to one another in the X-direction. Stated differently,for the components 510-511, the regions FR and FL share a border thatextends in the X-direction, whereas for the components 520-521, theregions FR and FL share a border that extends in the Y-direction.

The region FR or the region FL in each of the components 510-511 or520-521 may be configured to include the sub-patterns discussed above inFIG. 15A-15B, 17A-17B, or 18A-18E. Since the overlay measurement in theX-direction does not interfere with the overlay measurement in theY-direction (and vice versa), the regions FR and FL in the components510-511 may include different sub-patterns (or have differentsub-pattern designs) than the regions FR and FL in the components520-521, for example with respect to the sub-pattern's shape, size,number, and/or location.

Since the overlay mark 500 includes regions FR and FL for each of thecomponents 510-511 and 520-521, the overlay mark allows more overlaymeasurements to be made. For example, using the overlay calculationequations discussed above with reference to FIG. 4 , an overlay may bemeasured for the region FR, which may be expressed as OVL-FR herein,while another overlay may be measured for the region FL, which may beexpressed as OVL-FL herein. The OVL-FR and OVL-FL correspond to overlayin the same direction. For example, OVL-FR and OVL-FL could be overlayin the X-direction overlay if the regions FR and FL are implemented inthe components 510-511 of the overlay mark 500, or OVL-FR and OVL-FLcould be overlay in the Y-direction overlay if the regions FR and FL areimplemented in the components 520-521 of the overlay mark 500.

The overlay mark 500 can improve overlay measurement accuracy, sinceeach component 510-511 and 520-521 of the overlay mark 500 can obtaintwo overlay measurements (for a total of 8 overlay measurements for eachoverlay mark 500), whereas overlay marks without the regions FR and FLtypically make one overlay measurement per component (for a total of 4overlay measurements for each overlay mark). In some embodiments, theoverlay data OVL-FR and OVL-FL may be summed and then divided by 2 toobtain an average overlay, for each of the components 510-511 and520-521. In other words, an average overlay measurement may becalculated as OVL=(OVL-FR+OVL-FL)/2.

Due to the implementation of the sub-patterns—which cause the side-lobes580-581 shown in FIG. 16 —focus is also correlated with overlayaccording to various aspects of the present disclosure. For example,focus may be correlated with a difference between the overlay obtainedvia the region FR (e.g., OVL-FR) and the overlay obtained via the regionFL (e.g., OVL-FL). Mathematically, this may be expressed as follows:

Focus∝(OVL-F_(R)−OVL-F_(L))

In some embodiments, the correlative relationship between focus andoverlay may be approximated by a linear equation, such as Focus=a*OVL+b,where a and b are constants. In other embodiments, the correlativerelationship between focus and overlay may be expressed as a higherorder equation (such as one that includes a polynomial). Regardless ofthe equation used to approximate the correlative relationship betweenfocus and overlay, it is understood that once enough overlay samples arecollected, the constants (such as a and b) in the equation may besolved. With this equation at hand, focus may be determined once overlaydata is measured.

FIG. 19 illustrates a graph 600 of critical dimension (CD) versus focus.For example, the X-axis of the graph 600 represents focus (e.g., focuslength of an exposure process), and the Y-axis of the graph 600represents the measured CD. In more detail, one or more test wafers maybe used to produce the graph 600. A plurality of overlay marks similarto the overlay marks 500 may be implemented on the one or more testwafers. The focus (and/or exposure dose) that were used to fabricatethese overlay marks may vary somewhat from one another. As discussedabove, focus is correlated with overlay. Thus, the focus data associatedwith these different overlay marks on the wafer can be determined afterthe overlay data is obtained (e.g., via asymmetry light diffractionintensity as discussed above). The CD is measured corresponding to eachsample data of focus.

Note that the graph 600 includes a curve 610 and a curve 620. Thesecurves are referred to as “Bossung curves.” The curve 610 corresponds tothe measurements obtained via the components 510 and/or 511, whereas thecurve 620 corresponds to the measurements obtained via the components520 and/or 521. The curves 610 and 620 are displaced from one another interms of focus. This displacement may be intentionally implemented byconfiguring the sub-patterns on the components 510-511 to be differentfrom the patterns on the components 520-521, for example the size of thepatterns or the density of the patterns. Since each uniquedesign/arrangement of the sub-patterns corresponds to a unique curve, itfollows that the differences between the component 510/511 and thecomponent 520/521 (e.g., in terms of their sub-patterns) lead to theseparation of the curves 610 and 620 in FIG. 19 .

As shown in FIG. 19 , the curves 610 and 620 may intersect at anintersection point 640. According to various aspects of the presentdisclosure, the focus corresponding to the intersection point 640 mayrepresent an optimal focus. One reason for the intersection point 640being the optimal focus is that, the intersection point 640 represents afocus that would simultaneously optimize the CD for both the curve 610and the curve 620. Again, the curves 610 and 620 are obtained using thecomponents 510 and 511 that measure the X-direction overlay and theY-direction overlay, respectively. Therefore, optimizing the CDcorresponding to both curves 610 and 620 represents the best CDuniformity performance for features on the wafer in both the X and Ydirections. An optimal focus range 650 may also be obtained by expandingthe optimal focus from the intersection point 640 in either directionalong the X-axis in FIG. 19 , until the focus values that correspond tothe greatest CD values in the curves 610 and 620 are reached,respectively. This optimal focus range 650 may be used in futuresemiconductor fabrication to achieve optimized CD performance (e.g.,with respect to CD uniformity).

FIGS. 20A-20B illustrate a combo mark 700 and a compound mark 701according to embodiments of the present disclosure, respectively. Thecombo mark 700 and the compound mark 701 are different embodiments of anoverlay mark. They may each be similar to the overlay mark 500 discussedabove with reference to FIG. 14 . However, the combo mark 700 and thecompound mark 701 may each pertain to 3 or more layers. For example, thecombo mark 700 and the compound mark 701 may each include fourcomponents A, A′, and B, B′, which may be similar to the components510-511 and 520-521 discussed above. The difference between thecomponents A and A′ is that the component A corresponds to a +dshift/bias between its upper and lower layers, whereas the component A′corresponds to a −d shift/bias between its upper and lower layers.Similarly, the difference between the components B and B′ is that thecomponent B corresponds to a +d shift/bias between its upper and lowerlayers, whereas the component B′ corresponds to a −d shift/bias betweenits upper and lower layers.

For the combo mark 700, the components A and A′ are each configured tomeasure the overlay between a MD (e.g., Metal-zero) layer and apolysilicon layer. Since the alignment in the X-direction is ofimportance between the MD layer and the polysilicon layer, but theiralignment in the Y-direction is not of concern, the components A and A′are each configured to measure the overlay in the X-direction, but notin the Y-direction. Meanwhile, the components B and B′ are configured tomeasure the overlay between the MD layer and an OD (active region)layer. Since the alignment in the Y-direction is of importance betweenthe MD layer and the active region OD, but their alignment in theX-direction is not of concern, the components B and B′ are configured tomeasure the overlay in the Y-direction, but not in the X-direction.

The compound mark 701 shares many similarities with the combo mark 700,but with some minor differences. One difference is that the components Aand A′ in the compound mark 701 are configured to measure the overlaybetween a current layer and a pre-layer (e.g., below the current layer),and the components B and B′ in the compound mark 701 are configured tomeasure the overlay between a current layer and a post-layer (e.g.,above the current layer). The current layer, the pre-layer, and/or thepost-layer discussed herein is not limited to the metal-zero layer, thepolysilicon layer, or the active region OD layer, and they may be anylayer in semiconductor fabrication.

It is also understood that the components A, A′, B, and B′ of thecompound mark 701 are not restricted to measure overlay in just onedirection. For example, two different embodiments of the compound mark701 may both be implemented on a single wafer. In a first embodiment ofthe compound mark 701, its components A and A′ are each configured tomeasure the X-direction overlay, while its components B and B′ are eachconfigured to measure the Y-direction overlay. In a second embodiment ofthe compound mark 701, its components A and A′ are each configured tomeasure the Y-direction overlay, while its components B and B′ are eachconfigured to measure the X-direction overlay. As such, a plurality ofcompound marks 701 may be used to measure both the X-direction overlayand the Y-direction overlay between a current layer, a pre-layer belowthe current layer, and a post-layer above the current layer.

In addition to determining focus information, the overlay marks hereincan also be used to determine CD information according to variousaspects of the present disclosure. For example, referring now to FIGS.21A-21C, an overlay mark 800 is illustrated. The overlay mark 800 issimilar to the overlay mark 500 discussed above with reference to FIG.14 , as the overlay mark 800 also includes four components 810-811 and820-821, which are similar to the components 510-511 and 520-521 of theoverlay mark 500. Each of the components 810-811 and 820-821 alsoincludes an FR region and a FL region, again similar to the FR and FLregions of the overlay mark 500 discussed above.

In the illustrated embodiment herein, FIG. 21A illustrates the top viewof the configuration of the overlay mark 800 (e.g., with respect to itscomponents 810-811 and 820-821, as well the FR and FL regions therein),FIG. 21B illustrates the top view of one of the layers of the component810 (including its gratings 830 and sub-patterns 840), and FIG. 21Cillustrates the top view of one of the layers of the component 820(including its gratings 831 and sub-patterns 840) in the component 820.As discussed above, each of the components 810 and 820 may include anupper layer and a lower layer, where a bias/shift of +d or −d existsbetween the upper and lower layers. The sub-patterns 840 may be in oneof the layers, but not necessarily in the other layer. In the embodimentshown in FIGS. 21B and 21C, the upper layers are illustrated for thecomponents 810 and 820, and the sub-patterns 840 are located in theupper layers. However, it is understood that the sub-patterns 840 may beimplemented in the lower layers in another embodiment.

As shown in FIG. 21A, the component 810 is perpendicularly oriented withrespect to the component 820. This is illustrated in more detail inFIGS. 21B and 21C. For example, as shown in FIG. 21B that corresponds tothe component 810, the gratings 830 each extend in the Y-direction, andthe sub-patterns 840 are arranged into different columns that also eachextend in the Y-direction. Meanwhile, as shown in FIG. 21C thatcorresponds to the component 820, the gratings 831 each extend in theX-direction, and the sub-patterns 840 are arranged into different rowthat also each extend in the X-direction.

As discussed above, in order to best extract focus information from theoverlay mark 800, the sub-patterns 840 in FIG. 21B may be configureddifferently than the sub-patterns 840 in FIG. 21C, for example withrespect to their numbers, shapes, sizes, etc. In addition, since theoverlay mark 800 is used to extract CD information, the gratings 830 inFIG. 21B are configured differently than the gratings 831 in FIG. 21C aswell. For example, the gratings 830 in FIG. 21B each have a width 835measured in the X-direction, and the gratings 831 in FIG. 21C each havea width 836 measured in the Y-direction, where the width 835 is notequal to the width 836. In some embodiments, the width 835 may besubstantially smaller than the width 836, for example at least 25%smaller in some embodiments, or at least 50% smaller in otherembodiments. In other words, the gratings 831 in the component 820 aresubstantially “fatter” than the gratings 830 in the component 810. Thedifference in grating widths 835-836 between the gratings 830 and 831means that the components 810 and 820 of the overlay mark 800 havedifferent critical dimensions (CDs). In other words, the same overlaymark 800 may be configured such that different CDs exist.

While not specifically illustrated for reasons of simplicity, it isunderstood that the above discussions may also apply to the components811 and 821. In other words, the component 811 may be substantiallysimilar to the component 810, except that its upper and lower layers areshifted/biased in a different direction than the component 810.Likewise, the component 821 may be substantially similar to thecomponent 820, except that its upper and lower layers are shifted/biasedin a different direction than the component 820. As such, the component821 may also have substantially “fatter” gratings than the component 811(or than the component 810). Furthermore, it some other embodiments, thecomponents 811 and 821 may also be configured such that their gratingsmay have different widths than the gratings 830 and 831 discussed above.

The difference in the grating sizes between the various components ofthe overlay mark 800 allows for the extraction of CD information. Thisis illustrated in FIG. 22, which shows a graph 910 and a graph 920. Thegraph 910 has an X-axis that represents the critical dimension (CD), aswell as a Y-axis that represents diffraction intensity (I). The graph910 contains a plot 930 that illustrates a relationship between the CDand diffraction intensity, i.e., how they each vary in relation with oneanother. The graph 920 has an X-axis that represents the criticaldimension (CD), as well as a Y-axis that represents a derivative ofdiffraction intensity vs CD (dI/dCD). The graph 920 contains a plot 940that illustrates a relationship between the CD and dI/dCD, i.e., howthey each vary in relation with one another. In other words, the plot940 may be a derivative of the plot 930, or stated differently, the plot930 is an integral of the plot 940. It is understood that the plots 930and 940 illustrated herein are merely for illustrative purposes, andthat they may not be 100% mathematically accurate as drawn, ornecessarily reflect the true behavior of the underlying variables thatthey represent.

As discussed above, the overlay mark 800 is configured to have differentCDs (e.g., widths of the gratings for the various components of theoverlay mark 800). The diffraction light intensity may be measured foreach of the components of the overlay mark 800, and that yields todifferent points on the plot 930. It is understood that exposure energyis directly correlated with the diffraction light intensity. Thus, asexposure energy is varied, that may further change the relationshipbetween the diffraction light intensity and CD, which may correspond todifferent points on the plot 930. This may be repeated for one or moreoverlay marks similar to the overlay mark 800 in order to gather aplurality of measurements. In some embodiments, the plot 940 is obtainedfirst, for example by measuring CD and diffraction light intensityinformation using an SEM (scanning electron microscope) tool or anoverlay measurement tool, and then the plot 930 is derived by taking anintegral of the plot 940.

With the plot 930 available, CD predictions can be made. For example,the plots 930 and 940 may be generated or derived using one or moreoverlay marks 800 on a test wafer. After the plots 930 and 940 are madeavailable, the CDs of features on subsequently fabricated wafers may bepredicted by measuring the diffraction light intensity, for exampleusing overlay marks on the subsequently fabricated wafers. Since therelationship between the diffraction light intensity and the CD is nowknown (e.g., based on the plot 930), the CD may be determined via themeasurement of the diffraction light intensity. Accordingly, the overlaymarks discussed herein can be used to not only measure overlay (which isthe primary function of conventional overlay marks), but also to measurefocus information, as well as critical dimension (CD) information.

FIG. 23 illustrates a flowchart of a method 1000 of measuring focususing an overlay mark of the present disclosure. The overlay mark may bethe overlay marks 200A-200D, 400A-400F, 500, 700-701, or 800 discussedabove. The method 1000 includes a step 1010 and a step 1020, in which adiffraction light intensity is measured, for example in a manner similarto that discussed above with reference to FIG. 3 . In both the steps1010 and 1020, the diffraction light intensity is measured for the +1order diffracted light (e.g., I₊₁) and the I⁻¹ order diffracted light(e.g., I⁻¹). However, in step 1010, the diffraction light intensity ismeasured for a shift of +d between the upper and lower layers (e.g., asdiscussed above with reference to FIG. 4B), whereas in step 1020, thediffraction light intensity is measured for a shift of −d between theupper and lower layers (e.g., as discussed above with reference to FIG.4A). For example, the step 1010 may be performed using the components ofthe overlay mark that have a +d bias between its upper and lower layers,as in the case for components 210 or 221 of the overlay mark 200A ofFIG. 5 , whereas the step 1020 may be performed using the components ofthe overlay mark that have a −d bias between its upper and lower layers,as in the case for components 220 or 211 of the overlay mark 200A ofFIG. 5 .

The method 1000 includes steps 1030 and 1040, in which asymmetryinformation is obtained. The step 1030 is performed after the step 1010,where the +1 order and −1 order diffraction light intensity (measuredusing the shift +d) are used to calculate the asymmetry in the case of ashift of +d, which is expressed as As^(+d)=I₊₁−I⁻¹. The step 1040 isperformed after the step 1020, where the +1 order and −1 orderdiffraction light intensity (measured using the shift −d) are used tocalculate the asymmetry in the case of a shift of −d, which is expressedas As^(−d)=I₊₁−I⁻¹.

The method 1000 then proceeds to step 1050, in which overlay iscalculated based on the asymmetry information obtained in steps 1030 and1040. In an embodiment, the overlay is calculated using the followingequation:

${OVL} = {d \times \left( \frac{{As}^{+ d} + {As}^{- d}}{{As}^{+ d} - {As}^{- d}} \right)}$

where OVL represents the calculated overlay, and d represents themagnitude of the shift or bias between the upper and lower layers of theoverlay mark.

The method 1000 then proceeds to step 1060, in which the overlay isobtained for both the FR region and the FL region. As discussed above,the overlay marks 500 or 800 each have a plurality of components, whereeach component has an FR region and an FL region that is symmetrical tothe FR region (see FIGS. 15A-15B, for example). The overlay for the FRregion may be measured in accordance to steps 1010-1050 discussed above,and then the same procedure is repeated for the FL region. The overlaycorresponding to the FR region may be represented in FIG. 23 asOVL-F_(R), and the overlay corresponding to the FL region may berepresented in FIG. 23 as OVL-F_(L). Thus, at the end of step 1060, twooverlay measurements are obtained: one for the FR region, and the otherone for the FL region.

The method 1000 then proceeds to step 1070, in which the two overlaymeasurements OVL-F_(R) and OVL-F_(L) are averaged together to obtain amore accurate overlay. The step 1070 also extracts focus informationfrom the two overlay measurements OVL-F_(R) and OVL-F_(L). As discussedabove with reference to FIG. 16 , focus is also correlated with adifference between the FR region overlay and the FL region overlay, andit can be mathematically expressed as:

Focus∝(OVL-F_(R)−OVL-F_(L))

In this manner, the overlay marks of the present disclosure can be usednot to only measure overlay, they can also be used to extract focusinformation by performing the steps 1010-1070 of FIG. 23 .

FIG. 24 is a flowchart of a method 1500 of using an overlay mark inaccordance with some embodiments of the present disclosure.

The method 1500 includes a step 1510 of measuring a diffraction lightintensity via an overlay mark. The overlay mark includes an upper layerand a lower layer that is shifted with respect to the upper layer.

The method 1500 includes a step 1520 of determining, based on themeasured diffraction light intensity, asymmetry information associatedwith the overlay mark.

The method 1500 includes a step 1530 of calculating, based on thedetermined asymmetry information, an overlay associated with the overlaymark.

The method 1500 includes a step 1540 of performing at least one of thefollowing: extracting, based on the calculated overlay, a focusinformation of an exposure process; or extracting, based on thediffraction light intensity, a critical dimension information associatedwith a wafer.

In some embodiments, the extracting of the focus information comprisesdetermining an optimal focus range for performing the exposure process.In some embodiments, the extracting of the critical dimensioninformation comprises determining a relationship between a diffractionlight intensity and critical dimension. In some embodiments, the steps1510-1540 are performed on a test wafer, rather than on a productionwafer.

It is understood that additional processes may be performed before,during, or after the steps 1510-1540 of the method 1500. For example,the method 1500 may include a step of fabricating the overlay mark (or aplurality of similar overlay marks) on a wafer, such as a test wafer.The overlay mark may be fabricated before the step 1510 of measuring thediffraction light intensity. For reasons of simplicity, additional stepsare not discussed herein in detail.

FIG. 25 is a flowchart illustrating a method 1600 of semiconductorfabrication according to aspects of the present disclosure. The method1600 includes a step 1610 of patterning a first overlay mark on a wafer.The first overlay mark includes: a first component located in a firstregion of the first overlay mark, wherein the first component includes aplurality of gratings that extend in a first direction; a secondcomponent located in a second region of the first overlay mark, whereinthe second component includes a plurality of gratings that extend in thefirst direction; a third component located in a third region of thefirst overlay mark, wherein the third component includes a plurality ofgratings that extend in a second direction different from the firstdirection; and a fourth component located in a fourth region of thefirst overlay mark, wherein the fourth component includes a plurality ofgratings that extend in the second direction; wherein: the first regionis aligned with the second region; and the third region is aligned withthe fourth region. The method 1600 further includes a step 1620 ofperforming one or more semiconductor manufacturing processes using thefirst overlay mark.

In some embodiments, the first component, the second component, thethird component, and the fourth component each include an upper layer inwhich a first subset of the gratings are located and a lower layer inwhich a second subset of the gratings are located.

In some embodiments, for the first component, the first subset of thegratings are shifted with respect to the second subset of the gratingsby a distance +d in the second direction; for the second component, thefirst subset of the gratings are shifted with respect to the secondsubset of the gratings by a distance −d in the second direction; for thethird component, the first subset of the gratings are shifted withrespect to the second subset of the gratings by a distance −d in thefirst direction; and for the fourth component, the first subset of thegratings are shifted with respect to the second subset of the gratingsby a distance +d in the first direction.

In some embodiments, the first region, the second region, the thirdregion, and the fourth region correspond to four corner regions of asquare.

In some embodiments, at least a portion of the first overlay mark islocated adjacent to an edge of an integrated circuit chip or an edge ofa test line.

In some embodiments, the first overlay mark is a part of an apparatusthat further includes a pattern that includes at least a portion of anintegrated circuit chip, wherein the first overlay mark is disposedadjacent to a border of the pattern. In some embodiments, the apparatusfurther includes a second overlay mark that is disposed adjacent to acorner of the pattern, wherein the second overlay mark is configureddifferently from the first overlay mark. In some embodiments, the secondoverlay mark includes: a first component located in a first region ofthe second overlay mark, wherein the first component includes aplurality of gratings that extend in the first direction; a secondcomponent located in a second region of the second overlay mark, whereinthe second component includes a plurality of gratings that extend in thefirst direction, and wherein the first region and the second region arediagonally disposed with respect to one another; a third component and afourth component collectively located in a third region of the secondoverlay mark and share a border that extends in the first direction,wherein the third component and the fourth component each include aplurality of gratings that extend in the second direction; and a fifthcomponent and a sixth component collectively located in a fourth regionof the second overlay mark and share a border that extends diagonally inboth the first direction and the first direction, wherein the thirdregion and the fourth region are diagonally disposed with respect to oneanother, and wherein the fifth component and the sixth component eachinclude a plurality of gratings that extend in the second direction. Insome embodiments, the third component, the fourth component, the fifthcomponent, and the sixth component each include an upper layer in whicha first subset of the gratings are located and a lower layer in which asecond subset of the gratings are located; for the third component, thefirst subset of the gratings are shifted with respect to the secondsubset of the gratings by a distance +d in the first direction; for thefourth component, the first subset of the gratings are shifted withrespect to the second subset of the gratings by a distance −d in thefirst direction; for the fifth component, the first subset of thegratings are shifted with respect to the second subset of the gratingsby a distance +d in the first direction; and for the sixth component,the first subset of the gratings are shifted with respect to the secondsubset of the gratings by a distance −d in the first direction.

It is understood that additional processes may be performed before,during, or after the steps 1610-1620 of the method 1600. For reasons ofsimplicity, additional steps are not discussed herein in detail.

FIG. 26 is a flowchart illustrating a method 1700 of semiconductorfabrication according to aspects of the present disclosure. The method1700 includes a step 1710 of patterning an overlay mark on a wafer. Theoverlay mark includes: a plurality of first gratings disposed in a firstlayer of the overlay mark; a plurality of second gratings disposed in asecond layer of the overlay mark, wherein one of the first layer and thesecond layer includes an upper layer, and another one of the first layerand the second layer includes a lower layer disposed below the upperlayer; and a plurality of sub-patterns disposed in the first layer, butnot in the second layer, wherein a size of each of the first gratings isat least an order of magnitude larger than each of the sub-patterns. Themethod 1700 further includes a step 1720 of performing one or moresemiconductor manufacturing processes using the overlay mark.

In some embodiments, the first gratings and the second gratings eachextend in a first direction; and the first gratings are shifted withrespect to the second gratings in a second direction that is differentfrom the first direction.

In some embodiments, the first gratings, the second gratings, and thesub-patterns are located in a first region of the overlay mark; and theoverlay mark includes a second region that is symmetrical to the firstregion. In some embodiments, the overlay mark include a first componentand a second component; the first region and the second region areincluded in both the first component and the second component; the firstregion in the first component is oriented perpendicularly to the firstregion in the second component; and the second region in the firstcomponent is oriented perpendicularly to the second region in the secondcomponent. In some embodiments, the first component is configured tomeasure an X-direction overlay between a first layer and a second layerof a semiconductor device; and the second component is configured tomeasure a Y-direction overlay between the first layer and a third layerof the semiconductor device. In some embodiments, the sub-patterns inthe first component are configured differently from the sub-patterns inthe second component. In some embodiments, the first gratings in thefirst component and the first gratings in the second component havedifferent widths.

In some embodiments, the sub-patterns are arranged into a row or acolumn that extends in a same direction as each of the first gratings.

In some embodiments, the sub-patterns include at least a first subset ofsub-patterns and a second subset of sub-patterns; and the sub-patternsin the first subset have different top view profiles than thesub-patterns in the second subset.

It is understood that additional processes may be performed before,during, or after the steps 1710-1720 of the method 1700. For reasons ofsimplicity, additional steps are not discussed herein in detail.

FIGS. 27-32 illustrate diagrammatic cross-sectional side views of adevice 2000 at different stages of fabrication according to embodimentsof the present disclosure. The device 2000 may include an overlay markaccording to the various aspects of the present disclosure discussedabove. Referring now to FIG. 27 , a wafer 2100 is provided. For example,the wafer 2100 may include a silicon substrate in some embodiments, orit may include another semiconductor or dielectric material in otherembodiments. A layer 2110 is formed over the wafer 2100. The layer 2110includes a material that can be patterned, for example a dielectricmaterial or a metal material. A patterned layer 2120 is then formed overthe layer 2110. In some embodiments, the patterned layer 2120 is apatterned photoresist layer, which may be patterned by a firstphotomask. In other words, the patterned layer 2120 is defined by afirst photomask.

Referring now to FIG. 28 , an etching process is performed to the device2000 using the patterned photoresist layer 2120 as a mask. As a resultof the etching process, the layer 2110 is etched to assume the top viewpattern/shape of the patterned photoresist layer 2120. The patternedlayer 2110 may serve as the lower layer of the various overlay mark ofthe present disclosure discussed above.

Referring now to FIG. 29 , a layer 2150 is formed in the openingsdefined by the patterned layer 2110. A polishing process such as achemical mechanical polishing (CMP) process is then performed toplanarize the upper surfaces of the patterned layer 2110 and the layer2150. The layer 2150 includes a different material from the layer 2110.

Referring now to FIG. 30 , a layer 2210 is formed over the planarizedupper surfaces of the layers 2110 and 2150. The layer 2210 includes amaterial that can be patterned, for example a dielectric material or ametal material. A patterned layer 2220 is then formed over the layer2210. In some embodiments, the patterned layer 2220 is a patternedphotoresist layer, which may be patterned by a second photomaskdifferent from the first photomask (used to pattern the layer 2110). Inother words, the patterned layer 2220 is defined by a second photomask.In some embodiments, the patterned layer 2220 may correspond to theupper layer 100A discussed above with reference to FIG. 3 . For example,the patterned layer 2220 may be used to measure overlay with the layer2110, and if the measured overlay is not satisfactory, then thepatterned layer 2220 may be removed and replaced by another patternedlayer to improve overlay.

Referring now to FIG. 31 , an etching process is performed to the device2000 using the patterned photoresist layer 2220 as a mask. As a resultof the etching process, the layer 2210 is etched to assume the top viewpattern/shape of the patterned photoresist layer 2220. The patternedlayer 2210 may serve as the upper layer of the various overlay mark ofthe present disclosure discussed above. The patterned layer 2210 and thepatterned layer 2110 may also be used similar to the upper and lowerlayers in FIG. 3 to measure overlay.

Referring now to FIG. 32 , a layer 2250 is formed in the openingsdefined by the patterned layer 2210. A polishing process such as a CMPprocess is then performed to planarize the upper surfaces of thepatterned layer 2210 and the layer 2250. The layer 2250 includes adifferent material from the layer 2210. In some embodiments, the layers2150 and 2250 have the same material composition.

As this point, a dual-layered overlay mark is formed. Again, the upperlayer of the overlay mark is represented by the layer 2210, while thelower layer of the overlay mark is represented by the layer 2110. It isunderstood, however, that the overlay mark shown in FIG. 32 issimplified for ease of understanding. The layers 2210 and 2110 may eachinclude a plurality of patterns arranged in the manner described abovewith reference to FIGS. 3-21 , as well having a bias (e.g., +d or −d) ina given direction, as discussed above. In addition, it is understoodthat other functional circuit components (e.g., metal lines or vias) maybe patterned in the same layers as in the layers 2210 and 2110 of theoverlay mark. The overlay measured between the layers 2210 and 2110 mayindicate a degree of alignment between the functional circuitcomponents.

The overlay marks discussed herein may remain even after all thefabrication processes have been performed, and the wafer has been dicedand packaged into a plurality of IC chips. In other words, the overlaymarks of the present disclosure may appear or be detected in an IC chip.

In semiconductor fabrication, the overlay marks of the presentdisclosure may be used to measure the alignment between differentlayers. The degree of alignment between the upper and lower layers ofthe overlay mark represents the alignment between other patterns formedin the same layers as the upper and lower layers of the overlay mark,respectively. In some embodiments, the diffracted light intensities ofthe gratings in an overlay mark may be used to calculated the asymmetryinformation in order to calculate the alignment or overlay, for examplein a manner similar to that discussed above with reference to FIG. 3 .

It is understood that while the overlay marks discussed above may beused in EUV lithography processes to measure alignment between differentlayers in some embodiments, they are not limited to EUV lithography andmay be used in non-EUV lithography in other embodiments.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages over conventional methods. It isunderstood, however, that other embodiments may offer additionaladvantages, and not all advantages are necessarily disclosed herein, andthat no particular advantage is required for all embodiments. Oneadvantage is that the overlay marks of the present disclosure allow formore accurate overlay measurements to be made. For example, theconfiguration of the overlay marks discussed above in association withFIGS. 5-13 can reduce the effects of noise generated as a result of poorpattern density uniformity. Another advantage is that the overlay marksof the present disclosure allow for the extraction of focus information.For example, sub-patterns (e.g., the sub-patterns shown in FIGS.15A-15B, 17A-17B, and 18A-18E) are added to overlay marks, which lead tothe side lobes discussed above in FIG. 16 . The side lobes allow focusinformation to be extracted while not affecting overlay measurements. Anoptimal focus range may be determined, for example as discussed abovewith reference to FIG. 19 . Yet another advantage is that the overlaymarks of the present disclosure allows for the extraction of criticaldimension (CD) information. For example, as discussed above withreference to FIGS. 21A-21C, the components of the overlay mark areconfigured to have different grating widths (i.e., different criticaldimensions) to extract critical dimension information. In someembodiments, the extracted critical dimension information may include arelationship between critical dimension and diffraction light intensity(or exposure energy).

One embodiment of the present disclosure pertains to an apparatus. Theapparatus includes an overlay mark. The overlay mark includes: a firstcomponent located in a first region of the first overlay mark, whereinthe first component includes a plurality of gratings that extend in afirst direction; a second component located in a second region of thefirst overlay mark, wherein the second component includes a plurality ofgratings that extend in the first direction; a third component locatedin a third region of the first overlay mark, wherein the third componentincludes a plurality of gratings that extend in a second directiondifferent from the first direction; and a fourth component located in afourth region of the first overlay mark, wherein the fourth componentincludes a plurality of gratings that extend in the second direction.The first region is aligned with the second region. The third region isaligned with the fourth region.

Another embodiment of the present disclosure pertains to a method offabricating a semiconductor device. An overlay mark is patterned on awafer. The overlay mark includes: a plurality of first gratings disposedin a first layer of the overlay mark; a plurality of second gratingsdisposed in a second layer of the overlay mark, wherein one of the firstlayer and the second layer includes an upper layer, and another one ofthe first layer and the second layer includes a lower layer disposedbelow the upper layer; and a plurality of sub-patterns disposed in thefirst layer, but not in the second layer, wherein a size of each of thefirst gratings is at least an order of magnitude larger than each of thesub-patterns; and performing one or more semiconductor fabricationprocesses using the overlay mark.

Yet another embodiment of the present disclosure pertains to a method.The method includes: measuring a diffraction light intensity via anoverlay mark, wherein the overlay mark includes an upper layer and alower layer that is shifted with respect to the upper layer;determining, based on the measured diffraction light intensity,asymmetry information associated with the overlay mark; calculating,based on the determined asymmetry information, an overlay associatedwith the overlay mark; and performing at least one of the following:extracting, based on the calculated overlay, a focus information of anexposure process; or extracting, based on the diffraction lightintensity, a critical dimension information associated with a wafer.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A structure, comprising: a pattern of a portionof an integrated circuit (IC); and a plurality of marks disposed aroundthe pattern in a top view; wherein: each of the marks includes aplurality of gratings; a first subset of the gratings each extend in afirst direction in the top view; and a second subset of the gratingseach extend in a second direction in the top view, the second directionbeing different from the first direction.
 2. The structure of claim 1,wherein the first direction is perpendicular to the second direction. 3.The structure of claim 1, wherein the marks include overlay marks. 4.The structure of claim 1, wherein: the marks include a first subset ofmarks and a second subset of marks; each mark of the first subset ofmarks is disposed adjacent to a different edge of the pattern,respectively; each mark of the second subset of the marks is disposedadjacent to a different corner of the pattern, respectively; and thegratings in the first subset of marks are arranged differently than thegratings in the second subset of the marks.
 5. The structure of claim 4,wherein: each of the marks in the first subset includes a first region,a second region, a third region, and a fourth region; the first regionand the second region are disposed adjacent to one another in the firstdirection in the top view; the third region and the fourth region aredisposed adjacent to one another in the first direction in the top view;and the first region and the second region are disposed closer to thepattern than the third region and the fourth region in the top view. 6.The structure of claim 5, wherein: the gratings in the first region andthe second region each extends in the first direction in the top view;and the gratings in the third region and the fourth region each extendsin the second direction in the top view.
 7. The structure of claim 4,wherein: each of the marks in the second subset includes a first region,a second region, a third region, and a fourth region; the first regionand the second region are disposed adjacent to one another in the firstdirection in the top view; the third region and the fourth region aredisposed adjacent to one another in the first direction in the top view;and the first region and the second region are disposed closer to thepattern than the third region and the fourth region in the top view. 8.The structure of claim 7, wherein: the gratings in the first region andthe third region each extends in the first direction in the top view;and the gratings in the second region and the fourth region each extendsin the second direction in the top view.
 9. The structure of claim 1,wherein: at least some of the marks each have a first layer and a secondlayer; the first layer is disposed above or below the second layer; thefirst layer and the second layer are each divided into a first componentand a second component; for the first layer, the gratings in the firstcomponent are shifted with respect to the gratings in the secondcomponent; and for the second layer, the gratings in the first componentare unshifted with respect to the gratings in the second component. 10.The structure of claim 9, wherein the gratings in the first layer areshifted with respect to the gratings in the second layer.
 11. Thestructure of claim 9, wherein the first component and the secondcomponent share a border than extends in either the first direction orin the second direction.
 12. The structure of claim 9, wherein the firstcomponent and the second component share a border than extends in boththe first direction and the second direction.
 13. A structure,comprising: a pattern of a portion of an integrated circuit (IC); afirst overlay mark disposed next to an edge of the pattern in a topview; and a second overlay mark disposed next to a corner of the patternin a top view; wherein: the first overlay mark includes a plurality offirst gratings arranged according to a first configuration; and thesecond overlay mark includes a plurality of second gratings arrangedaccording to a second configuration different from the firstconfiguration.
 14. The structure of claim 13, wherein: the first overlaymark and the second overlay mark each includes a first quadrant, asecond quadrant, a third quadrant, and a fourth quadrant; the firstquadrant and the second quadrant share a first border than extends in afirst direction in the top view; the third quadrant and the fourthquadrant share a second border than extends in the first direction inthe top view; the first quadrant and the second quadrant collectivelyshare a third border with the third quadrant and the fourth quadrantcollectively, the third border extending in a second directionperpendicular to the first direction in the top view; and in at leastone of the first, second, third, or fourth quadrants, the first gratingsand the second gratings extend in different directions.
 15. Thestructure of claim 13, wherein: the first overlay mark or the secondoverlay mark includes an upper layer and a lower layer; and the firstgratings or the second gratings in the upper layer are offset withrespect to the first gratings or the second gratings in the lower layer.16. The structure of claim 15, wherein: the upper layer includes a firstregion and a second region; and the first gratings or the secondgratings in the first region are offset with respect to the firstgratings or the second gratings in the second region.
 17. A structure,comprising: a first pattern of a portion of an integrated circuit (IC);a second pattern that includes a plurality of first gratings; and athird pattern that includes a plurality of second gratings; wherein: thefirst gratings form a first top view profile for the second pattern; thesecond gratings form a second top view profile for the third pattern;and the first top view profile is different from the second top viewprofile.
 18. The structure of claim 17, wherein: the second patternincludes a first region, a second region, a third region, and a fourthregion; the first region and the third region are diagonally disposedwith respect to one another; the second region and the fourth region arediagonally disposed with respect to one another; the first gratings inthe first region and the second region extend in a same direction in atop view; the first gratings in the first region and the third regionextend in different directions in the top view; the third patternincludes a fifth region, a sixth region, a seventh region, and an eighthregion; the fifth region and the seventh region are diagonally disposedwith respect to one another; the sixth region and the eighth region arediagonally disposed with respect to one another; the second gratings inthe fifth region and the seventh region extend in the same direction ina top view; and the second gratings in the fifth region and the sixthregion extend in different directions in the top view.
 19. The structureof claim 18, wherein the second pattern is disposed closer to the firstpattern than the third pattern.
 20. The structure of claim 17, whereinthe second pattern is a first overlay mark and the third pattern is asecond overlay mark.